The photograph using silver halide has been heretofore widely used because of its excellent sensitivity and gradation as compared with those obtained by other photographic processes such as electrophotographic process and diazo photographic process. In this connection, a method of directly forming a positive image is known. According to this method, as described, for example, in U.S. Pat. No. 3,761,276 and JP-B-60-55821 (the term "JP-B" as used herein means an "examined Japanese patent publication"), an internal latent image-type direct positive silver halide photographic emulsion is used and a silver halide grain having formed in the inside thereof a latent image is developed with a surface developer (developer which substantially does not develop but leaves the latent image formed site inside the silver halide grain) while uniformly applying exposure or using a nucleating agent to obtain a positive image.
Conventionally, it is known that the microstructure of the silver halide crystal has an effect on the final photographic performance. Duffin, Photographic Emulsion Chemistry, p. 18, The Focal Press (1966) states that "In the case of silver iodobromide emulsion, an important factor to take account of is the position of iodide. The iodide can present mainly in the center of the crystal, can be distributed over the entire grain or can be present mainly on the outer surface. The actual position of the iodide is determined by the preparation conditions and the position apparently has an effect on the physical and chemical properties of the crystal."
In the so-called single jet method where iodide and bromide salts each in the whole amount are allowed to be present in a reaction vessel and an aqueous silver salt solution is introduced into the reaction vessel to produce silver iodobromide grains, silver iodide first precipitates, therefore, silver iodide is liable to concentrate in the center of the grain. On the other hand, in the double jet method where iodide and bromide salts both are simultaneously introduced together with silver salt into the reaction vessel, the distribution of silver iodide within the grain can be intentionally controlled. For example, silver iodide may be uniformly distributed throughout the grain or when the addition of bromide salt is reduced or stopped on the way of grain formation and the addition of iodide salt is continued, a silver iodide or a silver iodobromide shell having a high silver iodide content can be formed on the outer surface (outer side) of the grain. JP-A-58-113927 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a silver halide emulsion in which at least 50% of the entire projected area is occupied by silver iodobromide tabular grains having a thickness of less than 0.5 .mu.m, a diameter of 0.6 .mu.m or more and an average aspect ratio of 8:1 or more, the tabular grain has first and second opposing parallel main surfaces, and the emulsion contains tabular silver iodobromide such that a central region extending between these two main surfaces and the silver iodide content in the central region is lower than the iodide content in the region also extending between the two main surfaces but being displaced in at least one transverse direction. JP-A-59-99433 discloses a silver halide emulsion in which 10% (by number) or more of silver halide grains present in the silver halide emulsion are silver halide tabular grains having an aspect ratio of 5 or more, the emulsion contains a silver halide grain such that silver iodide is contained in the portion inner than the area having a silver amount of 80 mol % based on the silver amount of the entire grain, from the center part in the long or short axis direction of the grain (inner high iodide phase), the average iodide content in the inner high iodide phase is 5 times or more the average iodide content of the silver halide present on the outer side than the inner high iodide phase, and the silver amount of the inner high iodide phase is 50 mol % or less of the silver amount of the entire grain. Furthermore, JP-A-60-147727 discloses a silver halide photographic emulsion containing silver halide grains each having a multi-layer structure and an aspect ratio of 5 or less, in which the difference in the average iodide content between any two adjacent layers of the grain, each layer having a homogeneous iodide distribution, is 10 mol % or less and the total iodide content of the silver halide grain having a multi-layer structure is 20 mol % or less.
JP-A-60-14331 discloses a silver halide photographic emulsion containing silver halide grains each having a clear layer structure, in which the grain consists of a core part having a silver iodide content of from 10 to 45 mol % and a shell part having a silver iodide content of 5 mol % or less, and the grain has an average silver iodide content of 7 mol % or more. JP-A-61-245151 discloses a silver halide emulsion characterized in that the silver halide grain comprises a plurality of layers different in the silver iodide content, the outermost shell has a silver iodide content of 10 mol % or less, a high silver iodide content shell having a silver iodide content 6 mol % or more higher than that of the outermost shell is provided on the side inner than the outermost shell, and an intermediate shell having a medium silver iodide content is provided between the outermost shell and the high silver iodide content shell. According to the techniques described in these patent publications, the silver iodide content is varied depending on the position of individual grains (particularly between the inner side and the outer side of a grain) to thereby obtain good photographic properties.
Y. T. Tan and R. C. Baetzold submitted a report at the 41st Meeting of SPSE, where the energy state of silver halide is calculated and it is estimated that iodide in a silver iodobromide crystal grain has a tendency to form a cluster. In the above-described silver iodobromide tabular grains, the distribution of silver iodide is a change in the silver iodide content by the difference in the unit of at least from 300 to 1,000 .ANG., however, as estimated by Y. T. Tan and R. C. Baetzold, the silver iodobromide crystal is verified to have more microscopic inhomogeneous silver iodide distribution.
JP-A-4-107442 (corresponding to U.S. Pat. No. 5,206,134) discloses a method for producing a silver halide emulsion containing silver halide grains each controlled such that the iodide content on the surface of the silver halide grain is higher than the iodide content of a layer on the inner side, the grain having an iodide content of less than 1.0 mol % on average based on the entire grain, where in the formation of the grain surface, iodide is supplied in an amount of from 0.005 mol % to less than 0.3 mol % based on all silver halide grains by either (a) a method of simultaneously adding a silver nitrate solution and an iodide ion-containing solution or (b) a method of adding fine grain silver halide having an AgI and/or AgBrI composition so that the iodide content on the grain surface can be higher than that of the inner layer.
This technique has succeeded in obtaining a silver halide photographic emulsion having remarkably excellent development progressing property, superior sensitivity/fog ratio and high covering power for the tabular grain emulsion where particularly tabular grains have the same projected area diameter and the same thickness. However, the patent publication neither refers to an internal latent image-type direct positive silver halide emulsion nor teaches the effect thereof at all.